The speedometer and odometer in my '94 940 only works when I start driving after it sits overnight then quits after about the first 1/4 mile of driving. Do you think it is the sensor on the rear-end? The ABS light does not come on. I checked the wiring that come out of the sensor and the wires are not broken. The problem started after I had rear brakes and new rotors and calipers put on it, the installer hit the old rotors with a hammer straight on the end of the axles with a 3-5 lb. hammer to loosen the rotors. Would that damage the sensor on the rear-end by any chance? Or it might be a coincidence. Is there a chance of corrosion at the connections of the harness? What is the easiest way to troubleshoot the sensor?

 

the sensor is pretty darn rugged, its just a little coil of wire and a magnet, it picks up magnetic pulses from a 'tone ring' in the differential, I doubt the hammer blows did anything.

the wire from the sensor is a 'twisted pair' (two wires twisted around each other to keep noise out). this goes through a seal box in the trunk area, then forward along the drivers side of the door sills to a bulkhead connector, and directly to two pins on the back of the speedometer. these wires are brown or brown-white, and green-white, where the brown-white one is grounded at the speedometer. when the right rear wheel is spun, there should be a small A/C voltage on these wires.

 

99% of the time it's the speedo itself, the capacitors fail in all kinds of strange ways causing weird symptoms...